Deep Sea Fishes

The RV Investigator is currently undertaking oceanographic research along the 110°E meridian off Western Australia as part of the second International Indian Ocean Expedition. The voyage is led by Professor Lynnath Beckley of Murdoch University and the research is supported by a grant of sea time on RV Investigator from the CSIRO Marine National Facility.

Date: May 23, 2019

Time: 1200 AWST

Latitude: 30.5°S

Longitude: 110°E

Wind direction: E

Wind speed: 9.5 knots

Swell direction: W

Depth: 5392 m

Air temperature: 19°C

Sea temperature: 20°C

Notes: The night samples at Station 6 were jam-packed with phyllosoma (rock lobster) larvae, which had the zooplankton scientists grinning from ear to ear. The whole ship’s complement is enjoying the lovely calm weather.

Deep Sea Fishes

By Daniel Cohen and Dr M. Pilar Olivar

Deep-sea fishes reside at depths greater than 200 m beyond the effective influence of sunlight for most of the time. The mesopelagic zone (between 200–1000 m depths) is home to many diverse and bizarre fishes, such as dragonfishes, snaketooths, bristlemouths, hatchetfishes, lightfishes and lanternfishes. The early stages of these fishes develop in the epipelagic layer nearer to the surface, but juveniles and adults move down into the dark zone. Some families are characterized by performing diel vertical migrations through the water column (e.g., lanternfishes), while others (e.g., bristlemouths or hatchetfishes) remain in the dark part of the ocean all the time.

Species of the family Myctophidae (lanternfish) are the dominant migrating mesopelagic fishes, and they constitute the focus of our studies on this voyage. Adult myctophids are small (from 3 to 10 cm) and have some similarity in appearance to an anchovy, although being darker in color and characterized by the presence of small luminous organs on their bodies. Their larvae are small (from 2 to 15 mm) and transparent, similar to those of many other fish species. Myctophid larvae generally dominate plankton samples from the deep open ocean.

Worldwide, lanternfish comprise about 250 species and account for 65% of mesopelagic fish biomass. The name myctophid comes from the Greek word “mykter” meaning nose and “ophis” meaning serpent, while the common name lanternfish describes the array of light emitting organs called photophores that occur in species-specific patterns on their bodies.

Hatchet fish are the most well-recognised of the deep sea fishes, with their large over-sized jaws, bulgy eye and silver jacket. Photo: Micheline Jenner AM/CWR.

Living in the light-limited mesopelagic zone means little access to prey. Myctophids deal with this problem by performing diel vertical migration, being the movement up into the plankton rich surface waters at night to feed, before returning to the safety of the deep during the day. In this way, myctophids occupy an important position in the oceanic food web transporting organic matter vertically through their migrations and providing food for larger predatory species.

Despite their importance, relatively little is known about myctophids in the south-east Indian Ocean and this is largely due to the difficulty in sampling deep-sea fishes. Fortunately, thanks to their diel vertical migration at night, when they are feeding near the surface, these fishes can be captured using large nets towed from research vessels. Additionally, the presence of their larval stages nearer to the surface allows sampling by means of standard plankton nets, and an indirect way of studying species distribution, abundance and diversity. Lanternfishes, through their abundance, play a vital role in the health and functioning of the ocean, particularly because they are prey for commercially important species such as tuna, mackerel and squid and ecologically significant species such as seabirds and whales.